These studies are designed to study the factors which regulate the numbers of and control the differentiation of hematopoietic stem cells. Studies are being conducted in rabbits, irradiated mice, congenitally anemic mice, leukemic mice and in patients who receive therapeutic irradiation for non-hematologic disorders. The effects of transplanted granulocytic leukemia, mechanical marrow ablation and myelofibrosis on hematopoietic stem cells of blood, bone marrow and spleen are being studied.